Technical Field
The present invention relates to a calibration object for calibrating an image capturing device, and particularly to a conic section calibration object for calibrating an image capturing device, and which may provide a plurality sets of projection distance information.
Related Art
In a task for taking a picture on a full range special scene, a plurality of image capturing devices are used together, so that image information for any positions within the space may be obtained. At this time, a calibration process has to be undertaken on the image capturing devices to ascertain the image capturing devices may get in cooperation to produce appropriate information. The calibration refers to a process for deducing some inner parameters and outer parameters of the image capturing devices. To perform the calibration task, a calibration object is disposed outside the image capturing devices, so that the inner and outer parameters may be obtained. The image capturing each have their positions and azimuth angles, and which are defined with a Cartesian coordination system, since such coordination system may provide a system pointing characteristic for the image capturing devices with respect to the positions and the azimuth angles of the calibration object.
The inner parameters of the image capturing devices reflects a mapping relationship between a three dimensional target point in an image capturing device coordination system associated with the image capturing device and a two dimensional target image point corresponding thereto. On the other hand, the outer parameters of the image capturing devices refers to a rotational and shift relationship between a target point on a world coordinate system and an image capturing device coordination system. The common inner parameters of the image capturing devices include a focal distance and a position, a feature ratio of a pixel, a lens' distorted distortion projection center, and the like.
Generally, if the inner mechanism and the lens of the image capturing device does not vary in position, the inner parameters of the image capturing devices are fixed and not relevant to a disposition position of the image capturing devices. However, for the image capturing devices equipped with a zooming lens, the inner parameters, such as the focal distance, may vary with the different focal points. The outer parameters of the image capturing devices include the position and a photo-taking direction of the image capturing devices on the three dimensional coordination, including a rotation matrix and a shift matrix. As compared to the inner parameters, the outer parameters have the difference that their values are relevant to the disposition position and photo-taking direction of the image capturing devices, and thus they have to be re-calibrated again whenever the image capturing devices are moved once.
The conventional calibration object comprises a three dimensional calibration object, a two dimensional calibration object, and a one dimensional calibration object, and which are shown in FIG. 1A, FIG. 1B, and FIG. 1C, respectively.
For the calibration purpose, the three dimensional calibration object uses two or three flat surfaces perpendicular to one another or a flat object using a known and simple shift. This mechanism may achieve in a precise calibration result but require a precise mechanical shift platform, and is difficult to be applied onto a large range monitoring used image capturing devices.
The two dimensional calibration object has a pattern having a known distance, and such flat calibration object is used to calibrate the image capturing devices through a rotation and a shift of different directions, with only requiring a pattern having the calibration points printed out and pasted on a surface which may not be arbitrarily distorted. Thereafter, an image of the surface being subjected to the rotation and shift of different angles is taken and thus the parameters of the image capturing devices may be deduced. However, such calibration object has the main disadvantage that the calibration points information may not be observed when it is rotated to a certain angle.
A one dimensional calibration object has a know length, and may be used to evaluate the parameters of the image capturing devices by fixing one end while moving the other end of the calibration itself. When it is required to calibrate the outer parameters of a plurality of image capturing devices overlapping with each other, the calibration points have to be observed by a plurality of image capturing devices concurrently. However, it has the main disadvantage that it is required to fixed one end while move the other end to generate a plurality sets of calibration images.
The three dimensional type has a difficulty in real practice, while the one and two dimensional types suffers from a shading effect on the calibration points resulted from the flat surface. When the calibration object is rotated, different two of the calibration points may has a possibility of being shaded and thus lose its calibration effect.
In view of the above, it may be known that the prior art is still inherent with the issue of shading effect for the calibration used target points, and, therefore, a more ideal calibration mechanism for the image capturing devices is actually required.